Stories was a major growth driver in 2017, and even though this year's growth won't be as high as last year, the feature is still attracting new users to the platform.

For the most part, Instagram's user base skews younger, with millennials making up the largest share and representing nearly half of the platform's US user base this year.

It also skews female. Roughly 56% of female social network users will access Instagram in 2018, compared with 48.6% of male social network users.

"Instagram is succeeding in attracting new users of all ages," said eMarketer senior forecasting analyst Chris Bendtsen. "While Facebook has had trouble with teens and Snapchat with older generations, Instagram’s popularity is more balanced across all age groups."

"Among those three platforms this year, Instagram is the only one where the split of new users over the age of 35 vs. under 35 will be nearly 50-50," he added. "Instagram will add just more than 12 million new users this year, which is more than double any other social network. And while Facebook, Snapchat and Instagram all have a 'Stories' feature, it’s Instagram Stories that will be especially successful in capturing new sign-ups."